ABSTRACT Changes in the abundance and phenotypes of adipose tissue immune cells are a major determinant of systemic inflammation and insulin resistance during excess weight gain. Adipocyte expression of the class II major histocompatibility complex (MHCII) occurs early during high-fat diet (HFD) challenge and parallels pro- inflammatory changes in MHCII-activated CD4+ adipose resident T cells (ARTs) implicating the adipocyte as an instigator of obesity-induced inflammation. Adipocyte-specific MHCII null (aMHCII-/-) mice created to test this hypothesis 1) developed substantially less visceral adipose tissue (VAT) inflammation than their wild-type (WT) littermates when challenged with HFD, despite identical changes in body weight and %body fat, 2) had markedly more VAT regulatory T cells (Tregs), but not in other peripheral sites; 3) were more insulin sensitive with better glucose homeostasis and 4) when bred into an atherosclerosis prone LDLR-/- background, attenuated accelerated atherosclerosis without affecting plasma cholesterol and triglyceride levels. Tregs are a major component of the CD4+ ART population in lean mice, where they suppress inflammation to maintain normal VAT metabolism, but dramatically decrease during HFD-challenge. However, VAT Tregs are preserved in HFD-fed aMHCII-/- mice, which likely explains the improved metabolic and cardiovascular phenotype in these mice. The aMHCII-/- mutation, thus, provides a unique opportunity to specifically alter adipose inflammation, independent of obesity, dyslipidemia, and changes in peripheral T cells to investigate its impact on obesity- induced complications, particularly atherosclerosis. We hypothesize that decreased adipose inflammation attenuates atherosclerosis even in the presence of obesity. Specific Aims will address: 1) the effect of A) aMHCII-deficiency and B) visceral adipose tissue (VAT)-specific Treg depletion (via cells with defective VAT Treg homeostasis or an IL-33 receptor blocking antibody which inhibits IL-33-induced VAT Treg proliferation) on diet-induced atherosclerosis; 2) whether A) constitutive, adipocyte-specific MHCII overexpression promotes adipose inflammation to enhance atherosclerosis, and B) administration of IL-33 attenuates atherosclerosis through a VAT Treg-dependent mechanism; and 3) changes in the immune cell composition and molecular phenotypes in aortic lesions in mice with and without aMHCII mutations using T cell flow analyses of aorta, laser capture microdissection of plaque macrophages, and investigation of macrophage trafficking from VAT to aorta. The results of this investigation using adipocyte MHCII knock-in/knock-out models and several novel approaches to specifically alter VAT, but not peripheral, Tregs will determine the contributions of adipose tissue inflammation and VAT Tregs to the pathogenesis of obesity-associated atherosclerosis. This mechanistic insight sets the stage for development of better immune-based therapeutic strategies to combat CVD in the setting of obesity.